Our research is directed towards expanding the genetic tools available for the study of cultured mammalian cells and their virus. The genetic analysis of mammalian tissue culture cells has been quite difficult, in part because of the lack of a stringent conditional lethal system. For this reason we are attempting to isolate nonsense suppressors in cultured mammalian cells. The nonsense suppressor system would be valuable in the analysis of mammalian cells in culture and their viruses because it should be non-leaky and because it will permit direct biochemical identification of the mutant gene product. Among our collection of mutants we have identified cell lines containing ochre (UAA), amber (UAG) and opel (UGA) mutations in the HGPRT structural gene. We are in the process of screening revertants derived from these mutants for suppressor tRNA. The complexity of the mammalian cell can be greatly reduced by cloning. Cloned genes are further accessible to manipulations and transfer. For this reason we have been studying DNA-mediated gene transformation in cultured mammalian cells. Through coupling microinjection technology and construction of appropriate DNA vehicles transformation frequencies greater than 20% have been attained.